Correlates of behavioural dominance in mallards and American black ducks

American black duck, Anas rubripes, populations are thought to be in decline largely because of competitive exclusion by mallards, A. platyrhynchos, and/or introgressive hybridization. Mallard males reportedly are behaviourally dominant to black duck males, which may allow mallards to acquire black duck females as mates and/or exclude black ducks from high-quality breeding sites. The purpose of this study was to examine the influence of body size, body mass, degree of body symmetry and testosterone levels on dominance interactions between these two species, while controlling for effects of previous experience. Thirty dominance trials were performed in the winter and spring with groups of six males (three mallard and three black duck). Surprisingly, mallards were not generally dominant to black ducks. General linear model analyses revealed that morphological and hormonal variables were not consistently correlated with dominance, but place of origin and home pen dominance were. Pre-trial plasma testosterone concentration and change in testosterone concentration were significant effects in the winter trials, but in the opposite direction to that predicted. Size-adjusted body mass was correlated with dominance between Ontario mallards and wild-source black ducks in the spring. Dominance relationships between these two species are more complex than was originally thought. (C) 1996 The Association for the Study of Animal Behaviour